A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In almost all lithographic apparatus a level or height sensor is provided. This measures the position of the top surface of a substrate relative to a fixed reference to enable the substrate to be positioned at the correct vertical position (height or Z) and at the correct tilt angle (Rx & Ry) underneath the projection system during an exposure so that the mask image is correctly focused on the substrate across the image field. This process is generally referred to as leveling and may be performed “on-the-fly” or “off-axis”. In on-the-fly leveling, the level sensor measures the position of the top surface of the substrate directly underneath the projection system during or immediately before an exposure and a feedback loop adjusts the height and tilt of the substrate as necessary. In off-axis leveling, the surface contour of the substrate to be exposed is mapped in advance (usually by scanning the substrate below a level sensor located away from the optical axis of the projection system but in principle it could be performed with a level sensor mounted on axis) and set points for the substrate table height and tilt and/or adjustable elements of the projection system for an exposure or a series of exposures are calculated in advance.
There are various types of level sensor, including optical sensors and capacitive sensors. In optical sensors a light beam is directed onto the substrate and the reflected light detected. The vertical position of the substrate surface can then be derived in various ways, e.g. by determining the position of the reflected beam on a sensor. In capacitive sensors, the height of the substrate surface is detected using the fact that the capacitance between two surfaces is dependent on the distance between them. Further details of an off-axis leveling scheme and an optical level sensor are given in EP-A-1037117, which document is hereby incorporated in its entirety by reference.
In general, whether on-the-fly or off-axis leveling is used and whether optical or capacitive sensors are used, level sensing systems are configured to measure the height and/or tilt of several points on the substrate surface at the same time, in most cases using several separate sensing devices. In on-the-fly leveling this is necessary to derive tilt information and in off-axis leveling it reduces the time taken to generate the height map. It is therefore necessary to calibrate the different sensing devices relative to one another. Conventionally, this is done using a reference wafer that is assumed to be flat or whose surface profile is known accurately.